Up to now, in an atmospheric optical telecommunication system in which an information signal is transmitted frown an atmospheric optical telecommunication apparatus for reception by another atmospheric optical telecommunication apparatus via a light beam propagated through the atmosphere, especially when a laser is used as the light source, the laser light is transmitted from one of the atmospheric optical telecommunication apparatus after modulation in accordance with the information signal. The modulated laser light is received by the other atmospheric optical telecommunication apparatus and is demodulated to recover the information signal. In this way, the information signal is transmitted from one atmospheric optical telecommunication apparatus to another.
FIG. 1 illustrates bidirectional optical communication by laser light between the atmospheric optical telecommunication apparatus 81 and the atmospheric optical telecommunication apparatus 82 forming an atmospheric optical telecommunication system. Communication is achieved by the atmospheric optical telecommunication apparatus 81 or 82 transmitting a laser light via the lens 83 and by the atmospheric optical telecommunication apparatus 82 or 81 receiving the transmitted laser light via the lens 83. In the example shown, each of the atmospheric optical telecommunication apparatus includes an optical transmitter including a laser light source, and an optical receiver.
In an atmospheric optical telecommunication system employing a laser diode as the light source, a wavelength longer than 820 nm, for example, is used as the wavelength of the laser light. However, when wavelength multiplexing is used, the wavelength of the laser light may sometimes be shorter than 820 nm.
In atmospheric optical telecommunication systems for long-distance optical communication using a laser as the light source, changes in the properties of the atmospheric air that serves as the transmission medium result in variations in the carrier-to-noise ratio (C/N ratio) during signal transmission. The variations in the C/N ratio caused by the atmospheric air have up to now been thought to be produced by attenuation mainly caused by scattering, and by beam dancing caused by thermal fluctuations in the refractive index of the air.
The inventors have conducted experiments and have identified other noise-increasing factors in addition to scattering and beam dancing. These additional noise-increasing factors cause a marked increase in noise caused by atmospheric air, especially during long-distance transmission.
The first noise-increasing factor identified by the inventors' experiments is variations in the wavelength-dependent absorption caused by trace molecules in the atmosphere.
FIG. 2 shows the absorption spectrum of atmospheric air in the wavelength range of 770.0 nm to 841.6 nm, and shows that there are narrow absorption peaks in atmospheric air at a fairly large number of wavelengths in the wavelength range of 780 to 830 nm, which covers the most frequently used operating wavelengths of atmospheric optical telecommunication systems.
FIG. 3 illustrates the mechanism by which wavelength-dependent atmospheric absorption reduces the C/N ratio at the optical receiver. In the optical transmitter of the atmospheric optical telecommunication apparatus, a laser oscillating at a single frequency, such as a semiconductor laser operating in a single longitudinal mode, is used as the light source. The wavelength of the light generated by the laser can vary as a result of the temperature characteristics of the laser. If the wavelength of light generated by the semiconductor laser becomes coincident with one of the absorption lines in the wavelength absorption spectrum of atmospheric air as a result of the laser frequency shifting, the light power received by the optical receiver is reduced as a result of this atmospheric absorption, and the C/N ratio at the receiver is reduced. However, it has however been confirmed that, in long-distance atmospheric optical communication, an induced increase in noise occurs that far surpasses the deterioration in the C/N ratio resulting from the reduction in light power caused by absorption. Since the transmitted signal is attenuated by absorption, the C/N ratio of received signal is significantly worsened by synergistic effects.
Noise resulting from wavelength-dependent atmospheric absorption will from now on be called wavelength absorption noise. Wavelength absorption noise is usually produced as a result of wavelength shifting caused by the temperature characteristics of the laser. Thus, as the wavelength of the light generated by the laser changes relative to the wavelength of the atmospheric absorption peak, the wavelength absorption noise measured at the optical receiver gradually increases to a maximum level, and then decreases back to a normal level, as the temperature of the semiconductor laser in the transmitter increases. Since the wavelength absorption noise is caused by absorption by trace molecules in the atmosphere, its influence increases exponentially with increase of the transmitted distance.
It is thought that wavelength absorption noise is produced by the following two mechanisms. First, the laser frequency fluctuates slightly due to, for example, intensity modulation of the laser (this is generally termed "chirping"). Wavelength absorption noise is also slightly changed by the temperature characteristics or temporal changes of the laser oscillator. Thus, if the wavelength of the light generated by the laser overlaps the shoulder of one of the many wavelength-dependent absorption peaks in the atmosphere, as shown in FIG. 3, the shoulder of the adsorption peak translates the fluctuations in the laser frequency into fluctuations in the intensity of the light received by the optical receiver. Since the fluctuations in the laser frequency occur over a broad range of frequencies, the frequencies of the resulting fluctuations in intensity overlap the frequencies of the information signals amplitude modulated on the light beam. Accordingly, when the wavelength of the light generated by the laser coincides with the shoulder of an absorption peak, fluctuations in the laser frequency manifest themselves as intensity noise in the optical receiver.
In FIG. 3, S indicates an exemplary wavelength-dependent absorption peak in the atmosphere. Also shown in FIG. 3 is the emission spectrum of the light generated by the laser in the optical transmitter. If the wavelength .lambda. of the light generated by the laser is at the shoulder of the absorption peak S, where the atmospheric transmission factor decreases rapidly, and the wavelength of the laser light changes within a range of .DELTA..lambda. as a result of intensity modulation, and/or as a result of the temperature characteristics and/or temporal changes in the laser, the change .DELTA..lambda. in the wavelength .lambda. causes fluctuations in the intensity of the light received by the optical receiver. These fluctuations manifest themselves as intensity noise in the optical receiver, and lead to a considerable deterioration in the C/N ratio at the optical receiver.
Specifically, if the wavelength of the light generated by the laser in the optical transmitter coincides with one of the wavelength-dependent absorption peaks of the atmosphere, where the atmospheric transmission factor changes rapidly with wavelength, the light generated by the laser and having the wavelength .lambda. suffers a fixed loss, and the C/N ratio is lowered relatively insignificantly, so long as the wavelength of the light remains constant. However, if the wavelength of the light generated by the laser fluctuates due to temporal changes of the laser, these wavelength changes are translated into fluctuations in intensity by the wavelength-dependent characteristic of the absorption peak S shown in FIG. 3. These fluctuations in intensity manifest themselves at the optical receiver as intensity noise. For example, changes (.DELTA..lambda.) in the wavelength of the light generated by the laser as small as 0.1 .ANG. can degrade the C/N ratio by, for example, 10 dB or more.
In addition, since the changes in the wavelength of the light generated by the laser have an extremely high frequency, the wavelength absorption noise produced by this mechanism has broad frequency characteristics, such as those of white noise in the range of 0 to 400 MHz or 0 to 500 MHz. As a result, as mentioned above, the wavelength absorption noise overlaps the frequency regions occupied by the carriers of the information signals amplitude modulated on the light beam.
The second mechanism producing wavelength absorption noise is thought to be mode distribution noise, which generally explained as follows. If a semiconductor laser is operated in a pseudo-single mode, the distribution of output power between the main oscillation mode and the subsidiary oscillation mode may vary dynamically shown in FIGS. 4A and 4B, even though the total light output power of the semiconductor laser remains constant. In the main oscillation mode, light having a wavelength of .lambda..sub.0 is predominantly generated, as shown in FIG. 4A, whereas in the subsidiary oscillation mode, more light at wavelengths different from .lambda..sub.0 is generated, as shown in FIG. 4B. Transition between the oscillation modes shown in FIGS. 4A and 4B occurs very abruptly. Mode shifting produces no noise if the transmission ratio of the light power emitted by the optical transmitter and the light power received by the optical receiver remains independent of frequency, as in the case of the optical fiber transmission or near-distance atmospheric optical telecommunication.
However, if only the light generated in one of the oscillation modes, such as the main oscillation mode, reaches the optical receiver, and the power of the light generated by the main oscillation mode constantly fluctuates, the signal recovered in the optical receiver based on the light generated by such mode will be replete with noise and will constantly fluctuate in level. Light generated by a laser that is subject to mode shifting and transmitted via the atmosphere will result in wavelength absorption noise. For example, if the light generated by the main oscillation mode is significantly attenuated by an atmospheric absorption peak operating as a wavelength-dependent filter, then only the light generated by the subsidiary oscillation modes will be received, and the intensity of the light received will fluctuate constantly.
Thus, it is thought that the second mechanism producing wavelength absorption noise is fluctuations in the light power distribution between the main and subsidiary oscillation modes which, when subject to wavelength-dependent filtering by an atmospheric absorption peak, directly manifest themselves as wavelength absorption noise.
Thus, the mechanisms by which wavelength absorption noise is generated are thought to be noise produced by subjecting changes in the frequency of the laser to a sharp atmospheric absorption peak, and/or mode distribution noise accompanying the fluctuations in the laser operating mode in conjunction with atmospheric absorption. However, the cause of this noise cannot always be identified precisely. The dynamic noise resulting from these mechanisms is called the wavelength absorption noise to distinguish this noise from a second noise increasing factor, which will now be described.
The absorption spectrum resulting from trace molecules in the air is already known. This absorption spectrum is described in, for example, ATTENUATION OF VISIBLE AND INFRARED RADIATION, CCIR, Rep. 833, 1982. However, the magnitude of this absorption is low, with a maximum in the order of a maximum of 3 dB when measured with a spectroscope having the resolution of approximately 1A over a distance of about 10 km in atmospheric air on a fine autumn day. Thus, the wavelength-dependent absorption characteristics of the atmosphere have not been regarded as an important factor in determining the performance of an atmospheric optical telecommunication system for ground use over a distance of 1 to 2 km. It has been thought that the probability of the light wavelength employed in an atmospheric optical telecommunication system falling in the absorption range is not high, so that the existence of such wavelength-dependent absorption has been disregarded with respect to atmospheric optical telecommunication systems. Even although the wavelength range of the light used in such systems substantially overlaps the wavelength range of the absorption spectrum, it has been assumed that there is always a small difference between the wavelength of the light generated by the laser and that of the absorption peaks in the absorption spectrum, so that atmospheric absorption would not unduly affect the light received by the optical receiver.
The present inventors have also confirmed that, if a laser operating in the plural modes described above is used as the light source in a long-distance atmospheric optical telecommunication system, steady-state noise is produced by the second factor producing the increase in noise. It is thought that this steady-state noise is a result of the laser operating in more than one oscillation mode. This noise is produced even if the distribution of light power between the different oscillation modes remains static.
Specifically, as shown in FIG. 5, in a long-distance atmospheric optical telecommunication system, the light beam BD, arriving at the optical receiver, is generally larger in diameter than the light receiving lens 83 of the optical receiver of the atmospheric optical telecommunication apparatus 81 or 82. In addition, in the plane of the light beam BD perpendicular to the optical axis, the light intensity changes spatially due to fluctuations in the refractive index of the atmosphere. This produces portions of different light intensity similar to interference fringes. Because of the dimensional relationship between the received light beam BD and the light receiving lens 83, the optical receiver receives only the bright portions bp (corresponding to the bright bands of interference fringes) among the light portions of different light intensities. In other words, the optical receiver receives only portions of the incident light beam (e.g., the bright portions bp). The location of the intensity variations in the incident light beam depends on the wavelength of the light, so that the optical receiver operates in effect as a wavelength filter. In FIG. 5, portions dp shown by hatching correspond to the dark bands of the interference fringes.
Since interference fringes are generally formed as a function of the wavelength of the light, it may occur that, even though the portions of the light beam BD received by the light-receiving lens of the optical receiver correspond to the bright portions bp of the interference fringes in the light generated by the main oscillation mode of the laser, those portions correspond to the dark portions dp of the interference fringes in the light generated by the subsidiary oscillation mode of the laser. In this case, a significant steady-state noise is observed, thereby worsening the C/N ratio of received signals.
The noise resulting from the noise generating mechanism just described will be called steady-state mode distribution noise, to distinguish it from the mode distribution noise caused by fluctuations in the laser oscillation wavelength thought to produce the wavelength absorption noise. The steady-state mode distribution noise is mainly observed when the laser is operated in a pseudo-single mode, intermediate between a completely single mode and a completely multiple mode. It is not observed when the laser is operated in a completely single mode, nor when the laser is operated in a completely multiple mode.
Structurally, semiconductor lasers are classified into gain waveguide lasers and refractive index waveguide lasers. Gain waveguide lasers operate in the pseudo-single mode, so that they are susceptible to the above-mentioned steady-state mode distribution noise. Refractive index waveguide lasers are usually operated in the single mode, but will occasionally produce steady-state mode distribution noise when operated at a low output power, or due to mode competition (a transient unstable state produced at the time of transition from one output state to another).
The third factor responsible for increased noise is noise due to the above-mentioned first or second factors induced by the return light. Specifically, since the light beam arriving at the optical receiver in an atmospheric optical telecommunication system constantly fluctuates, the return light reflected back from the lens surface or the surface of the light-receiving device of the optical receiver is subject to constant significant changes in intensity. In a semiconductor laser, the longitudinal oscillation mode may undergo skipping, or the laser may operate in plural modes, as a result of the presence of the return light.
As a result, in an atmospheric optical telecommunication system in which the state of the return light constantly changes, a semiconductor laser that is susceptible to the influence of the return light cannot maintain its stable single-mode oscillation, but undergoes mode competition or mode hopping frequently. Mode competition leads to the mode distribution noise as mentioned above. In addition, if one of the competing modes coincides with one of the absorption peaks in the atmosphere, wavelength absorption noise will be produced. Return light tends to produce wavelength absorption noise due to abrupt mode hopping, and hence is observed as being an unstable state in which noise is produced in intermittent bursts.
The inventors have confirmed that the above-mentioned noise increasing factors occasionally lead to a noise increase that is so severe that it can cause a total failure of a long-distance atmospheric optical communication.
In a conventional atmospheric optical telecommunication system, as described above. high-quality atmospheric optical communication may occasionally be impossible as result of the above-mentioned noise-increasing factors. However, it is desirable that high-quality atmospheric optical communication be feasible at all times. Thus, it is desirable to prevent the above-described noise increasing factors from resulting in an unacceptable increase in the noise level.